DESCRIPTION

The SETTRANSACTION command sets the characteristics of the current
transaction. It has no effect on any subsequent transactions. SETSESSIONCHARACTERISTICS sets the default transaction characteristics
for subsequent transactions of a session. These defaults can be
overridden by SETTRANSACTION for an individual transaction.
The available transaction characteristics are the transaction isolation
level, the transaction access mode (read/write or read-only), and the
deferrable mode. In addition, a snapshot can be selected, though only
for the current transaction, not as a session default.
The isolation level of a transaction determines what data the
transaction can see when other transactions are running concurrently:
READ COMMITTED
A statement can only see rows committed before it began. This is
the default.
REPEATABLE READ
All statements of the current transaction can only see rows
committed before the first query or data-modification statement was
executed in this transaction.
SERIALIZABLE
All statements of the current transaction can only see rows
committed before the first query or data-modification statement was
executed in this transaction. If a pattern of reads and writes
among concurrent serializable transactions would create a situation
which could not have occurred for any serial (one-at-a-time)
execution of those transactions, one of them will be rolled back
with a serialization_failure error.
The SQL standard defines one additional level, READ UNCOMMITTED. In
PostgreSQL READ UNCOMMITTED is treated as READ COMMITTED.
The transaction isolation level cannot be changed after the first query
or data-modification statement (SELECT, INSERT, DELETE, UPDATE, FETCH,
or COPY) of a transaction has been executed. See Chapter 13,
Concurrency Control, in the documentation for more information about
transaction isolation and concurrency control.
The transaction access mode determines whether the transaction is
read/write or read-only. Read/write is the default. When a transaction
is read-only, the following SQL commands are disallowed: INSERT,
UPDATE, DELETE, and COPY FROM if the table they would write to is not a
temporary table; all CREATE, ALTER, and DROP commands; COMMENT, GRANT,
REVOKE, TRUNCATE; and EXPLAIN ANALYZE and EXECUTE if the command they
would execute is among those listed. This is a high-level notion of
read-only that does not prevent all writes to disk.
The DEFERRABLE transaction property has no effect unless the
transaction is also SERIALIZABLE and READ ONLY. When all three of these
properties are selected for a transaction, the transaction may block
when first acquiring its snapshot, after which it is able to run
without the normal overhead of a SERIALIZABLE transaction and without
any risk of contributing to or being canceled by a serialization
failure. This mode is well suited for long-running reports or backups.
The SET TRANSACTION SNAPSHOT command allows a new transaction to run
with the same snapshot as an existing transaction. The pre-existing
transaction must have exported its snapshot with the pg_export_snapshot
function (see Section 9.26.5, "Snapshot Synchronization Functions", in
the documentation). That function returns a snapshot identifier, which
must be given to SET TRANSACTION SNAPSHOT to specify which snapshot is
to be imported. The identifier must be written as a string literal in
this command, for example '000003A1-1'. SET TRANSACTION SNAPSHOT can
only be executed at the start of a transaction, before the first query
or data-modification statement (SELECT, INSERT, DELETE, UPDATE, FETCH,
or COPY) of the transaction. Furthermore, the transaction must already
be set to SERIALIZABLE or REPEATABLE READ isolation level (otherwise,
the snapshot would be discarded immediately, since READ COMMITTED mode
takes a new snapshot for each command). If the importing transaction
uses SERIALIZABLE isolation level, then the transaction that exported
the snapshot must also use that isolation level. Also, a non-read-only
serializable transaction cannot import a snapshot from a read-only
transaction.

NOTES

If SETTRANSACTION is executed without a prior STARTTRANSACTION or
BEGIN, it emits a warning and otherwise has no effect.
It is possible to dispense with SETTRANSACTION by instead specifying
the desired transaction_modes in BEGIN or STARTTRANSACTION. But that
option is not available for SETTRANSACTIONSNAPSHOT.
The session default transaction modes can also be set by setting the
configuration parameters default_transaction_isolation,
default_transaction_read_only, and default_transaction_deferrable. (In
fact SETSESSIONCHARACTERISTICS is just a verbose equivalent for
setting these variables with SET.) This means the defaults can be set
in the configuration file, via ALTERDATABASE, etc. Consult Chapter 18,
Server Configuration, in the documentation for more information.

EXAMPLES

To begin a new transaction with the same snapshot as an already
existing transaction, first export the snapshot from the existing
transaction. That will return the snapshot identifier, for example:
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT pg_export_snapshot();
pg_export_snapshot
--------------------
000003A1-1
(1 row)
Then give the snapshot identifier in a SETTRANSACTIONSNAPSHOT command
at the beginning of the newly opened transaction:
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SET TRANSACTION SNAPSHOT '000003A1-1';

COMPATIBILITY

These commands are defined in the SQL standard, except for the
DEFERRABLE transaction mode and the SETTRANSACTIONSNAPSHOT form,
which are PostgreSQL extensions.
SERIALIZABLE is the default transaction isolation level in the
standard. In PostgreSQL the default is ordinarily READ COMMITTED, but
you can change it as mentioned above.
In the SQL standard, there is one other transaction characteristic that
can be set with these commands: the size of the diagnostics area. This
concept is specific to embedded SQL, and therefore is not implemented
in the PostgreSQL server.
The SQL standard requires commas between successive transaction_modes,
but for historical reasons PostgreSQL allows the commas to be omitted.
PostgreSQL 9.5.0 2016 SET TRANSACTION(7)